Recently purchased a 2004 xc90. 143k miles. Took it back because of some noises (but no vibration) and they replaced the left front axle. 2 days later while driving on the interstate, it started vibrating really bad and we thought we had a flat tire. Pulled off the road but no flat. Started driving again and at first it seemed ok but then noticed again but just slight so we thought it was grooves in the road. Then after a few more min we were on smooth pavement and it started again. It felt like the entire bottom of the car was going to fall out. The front drivers side tire seems to have a noise that comes and goes. Vibration comes and goes but only after about 35-40mph.

 

Drive axles

 

Take it back to where you had the repairs done.

 

Had a problem like that with mine. The solution was new tires.

The old tires suffered from tread separation. They held air and looked normal, but the car felt like it was wallowing or swimming. It was worst while accelerating from a stop.

That was the first time I ever felt such a thing and diagnosing it took some time. I spent an hour under the car looking at every suspension and steering joint and came up empty handed. The good guys at my local tire shop figured it out. Without them I would have sold the car, that's how frustrated I was.